One of the problems in the spread spectrum (hereinbelow abbreviated to SS) communication system using a wide frequency band is that impossibility of communication or error rate is increased by narrow band interference of high level. In order to solve this problem, a filter using surface acoustic wave (hereinbelow abbreviated to SAW) has been invented.
As means for solving the problem described above a Japanese patent, application No. 1-267503 (corresponds to U.S. Ser. No. 07/521 142), has filed by the inventors of the present invention, etc.
FIGS. 6A and 6B indicate the construction of an SAW element corresponding to 1 channel having a certain central frequency in a device for removing narrow band interference signals according to the invention described in the previous application stated above.
In the figures, reference numeral 1 is a p.sup.+ (n.sup.+) type Si monocrystal substrate; 2 is a p(n) type Si epitaxial layer formed on the substrate; 3 is a thermal oxide layer formed further thereon; 4 is a ZnO piezoelectric thin layer formed on the thermal oxide layer. Said Si substrate, Si epitaxial layer 2, the thermal oxide layer 3, the piezoelectric layer 4 constitutes a piezoelectric substrate 5, 6 and 7 are metal electrodes formed on the layer 4, which are an input SAW interdigital transducer, an output SAW interdigital transducer and a gate electrode, respectively; 8 is a high impurity concentration diffusion region formed in the p(n) type Si epitaxial layer 2 under each of the metal electrodes serving as the transducer, which plays the role of increasing the excitation efficiency of the transducers; and 9 is an n.sup.+ (p.sup.+) type impurity diffusion region formed in the p(n) type Si epitaxial layer 2 under the gate electrode 7, in which a first pn diode array is formed along the SAW propagation path. As indicated in the older patent application stated above, the operation of this pn diode array plays the role of controlling the carrier density in the epitaxial layer by controlling the diode bias and varying the attenuation constant of the SAW above 100 dB/cm by the interaction between the SAW and carriers. That is, it has a function of performing turning on and off of channels with a high speed. 10 is an n(p) type impurity diffusion region formed in the p(n) type Si epitaxial layer 2 outside of the input transducer, in which a second pn diode array 10' is formed; 11 is a resistor connected with each of the pn diode arrays; 12 is a DC power supply; 13 is a monitor terminal for voltage signals obtained by transforming input signals into SAW and detecting them by means of the second pn diode array 10', which is a terminal through which the intensity (electric power) of the input signal within the relevant channel (frequency domain) is observed as variations in voltage; and 14 is a bias control terminal for the first pn diode array.
FIGS. 7A and 7B show another embodiment of the older invention stated above. The operation thereof is similar to that explained, referring to FIGS. 6A and 6B and the difference is that a high resistance Si monocrystal substrate 15 is used as the substrate and the p.sup.+ (n.sup.+) type impurity diffusion region 16 is formed between the n+(p+) impurity diffusion regions in the p(n) type Si epitaxial layer 2. That is, the difference is that the ohmic contact for the p+(n+) type region is formed on the back side surface in FIGS. 6A and 6B, while it is formed on the front side surface in FIGS. 7A and 7B. The construction indicated in FIGS. 7A and 7B has an advantage that the pitch of the pn diode arrays can be designed independently from the thickness of the epitaxial layer and has a performance higher than that indicated in FIGS. 6A and 6B as the filter construction.
Now the function of detecting the SAW signal by means of the second pn diode array described above will be explained. The second pn diode array 10 disposed outside of the input transducer 5 is biased by the DC power supply through the resistor 11. The bias point giving the optimum sensitivity for the SAW is at a point, where the diode array is a little forward biased The SAW, into which the input signal is transformed by the input transducer, propagates on the second pn diode array to modulate the potential of the diodes in space and in time. A DC component depending on the signal intensity of the SAW is produced by the non-linear resistance of the second pn diodes and the potential at the monitor terminal 13 is shifted from the initial bias voltage to an inverse bias. The amount of the shift corresponds to the intensity of the input signal.
FIG. 8 indicates the relation between the electric power of the input signal and the amount of the bias shift. The amount of the bias shift is proportional to the square of the electric power of the SAW (electric power of the input signal). In this way the potential of the SAW is square-detected by the pn diode array disposed outside of the input transducer and thus the intensity of the input signal is obtained as the base band signal.
Consequently it is possible to obtain easily information on the distribution of the frequency spectre intensity of the input signal by constructing the device by connecting a plurality of channels consisting of the SAW element described above having different central frequencies in parallel.
FIG. 9 shows an example of the construction of a narrow band interference suppressing filter composed of n channels connected in parallel consisting of SAW elements having the construction described above. Hereinbelow this narrow band interference suppressing filter is called AISF (Adaptic Interference Suppression Filter).
The part composed of a group of input transducers 17 and a group of second pn diode arrays 20 in FIG. 9 is the part, which monitors the intensity distribution of the input spectrum. The groups of input and output transducers 17 and 18 perform the function of a sorting filter, by which input signals are classified, depending on the frequency, propagated and again synthesized. The group of the first pn diode arrays 19 on the SAW propagation path disposed for every channel controls the attenuation constant of the SAW of each channel.
The operation of the AISF system indicated in FIG. 9 is as follows. The input signal is transformed into the SAW by the group of input transducers 17 and the amount of the bias shift for the group of second pn diode arrays 20 is monitored. A bias control circuit 21 controls the bias voltage for the first pn diodes for the propagation control, depending on the amount of the bias shift thus obtained.
The function of this bias control circuit 21 is to amplify the amount of the bias shift of the second pn diodes 20 depending on the signal intensity for different channels and to bias backward pn diodes 19 in channels, in which the amount of the shift is great. Or not only it amplifies the amount of the bias shift, but also it has a function of setting a certain threshold and turning on (forward bias) and off (backward bias) of the bias for the first pn diode 19 by means of a comparator.
As it can be seen from FIGS. 6A, 6B, 7A, 7B and 9, the AISF system indicated in FIG. 9 has an advantage that it can be integrated on a same Si substrate in a monolithic manner. By using it, it is possible to intend to improve the performance and to decrease the size of the system.
FIGS. 10(a), 10(b) and 10(c) are schemes for explaining the flow of the signal processing by the AISF system, in which FIG. 10(a) indicates the spectre of an input signal, in which narrow band interference waves B and C are added to a wide band SS-SD signal A.
The channel numbers k and m corresponding to the frequencies of the interference waves B and C are detected and adapted to this environment, the filter characteristics of the AISF is changed to that indicated in FIG. 10(b), in which notches are formed in the channel portions k and m. The spectrum of the output signal of the AISF having the characteristics is that indicated in FIG. 10(c), in which the interference waves B and C are suppressed.
FIG. 11 shows an example of the system construction, in which the AISF system indicated in FIG. 9 is incorporated in the input stage of the receiving section in a DS-SS system. The AISF system 30 is disposed in the stage preceding a correlator and an AGC (automatic gain control) circuit 31' is disposed in the stage further preceding it. In this example of the system construction the output of the AISF system 30 is fed back to effect the gain control of the amplifier by means of the AGC 31'. BPF 32' is a band pass filter. If the AGC circuit were disposed without AISF system, when there are narrow band interference waves of high electric power, since the gain control is effected by the interference signals themselves, this would give rise to a problem that communication is impossible, or error rates increases, etc. in the DS-SS communication system. On the contrary, if the AISF system 30 is disposed, the gain control can be effected, depending on the signal intensity of a signal, for which the interference waves are suppressed, i.e. the spread spectrum signal itself and therefor the function of the DS-SS system is increased remarkably.
Further, FIGS. 12A, 12B and 13A, 13B show modified examples of the embodiments indicated in FIGS. 6A, 6B and 7A, 7B, in which a gate electrode 7' is disposed on the ZnO piezoelectric thin layer, corresponding to the second diode array 10.
Now the invention disclosed in the older application has still several points to be improved, as described below.
That is, in FIG. 9, since the detection sensitivity of the group of second pn diode array 20 monitoring the intensity distribution of the input spectrum is low, no good S/N can be obtained at the output thereof. For this reason the bias control circuit 21 was operated erroneously because of influences of noise. Therefore, in order to be able to reduce the influences of noise, the inventors of the present invention has proposed in Japanese patent application No. 1-337839 (which corresponds to U.S. Ser. No. 07/521 142) an ASIF, in which a third group of diode arrays having a shape approximately identical to that of the second pn diode arrays is disposed outside of the propagation path of the SAW.
According to this proposition, since the third group of pn diode arrays serves as a reference signal source, the influences of noise can be reduced by forming the difference between the voltage thereof and the potential of the second group of pn diode arrays.
FIG. 14 shows an embodiment of the device for removing narrow band interference signals according to the older patent application described previously, in which the same reference numerals as those indicated in FIG. 9 represent identical or analogous means.
In the device indicated in FIG. 14 the construction different from that disclosed in the older patent application indicated in FIG. 9 consists in that a third pn diode array 20' is disposed. This diode array 20' has a shape approximately identical to the second pn diode array 20 and is disposed on a same SAW element chip outside of the SAW propagation path. The detection terminal for the third pn diode array 20' is taken out independently from that for the second pn diode array to be connected with the bias control circuit 21 and connected also with the DC power supply through the resistor 11.
In the bias control circuit 21, there is disposed e.g. a comparing circuit 22 as indicated in FIG. 16. The detection signals coming from the terminals for detecting of the second group of pn diode arrays 20 are applied to the + terminals thereof and the detection signal coming from the terminal for detection of the third pn diode array 20' is applied to the terminal.
Since the third pn diode array 20' is disposed outside of the propagation path of the SAW, it can act as the reference signal source.
Consequently the potential difference between the second and the third pn diode array is formed in the bias control circuit 21 by the comparing circuit 22 and the processing is effected by using this difference signal as an effective detection signal. In this way, if this difference signal is used as the detection signal, the influences of noise can be reduced.
FIGS. 15A, 15B and 15C are schemes indicating an example of the SAW element described above, in which FIG. 15B is a top view of the element; FIG. 15A is a cross sectional view along the line A--A'; and FIG. 15B is a cross sectional view along the line B--B'. In FIGS. 15A, 15B and 15C, reference numeral 10' represents an n(p) impurity diffusion region formed in the p(n) type Si epitaxial layer 2, which constitutes the third diode array and 7" indicates a gate electrode.
The device can be operated, even if there is no gate electrode 7' indicated in FIG. 15.
However no concrete construction of the bias control circuit suitable for the AISF as described above and indicated in FIGS. 9 and 14 has been proposed and development thereof has been strongly desired.